For several reasons, the tandem trailer truck concept has become an essential part of any large scale cargo movement over the highway. Furthermore, the idea that one crew and one tractor can tow two or more trailers is attractive, not only because of the increased volume of goods shipped by truck and the increasing cost of individual crews and tractors, but also because the use of several smaller trailers rather than one large trailer is more compatible with modern "cargo module" systems. The cargo module system makes use of a single cargo container which is easily transferable between transportation means.
One important drawback to tandem trailer arrangements has been the problem of safety. A single tractor and trailer loaded with 30 tons of cargo and traveling at 70 miles per hour represents a serious danger no matter how carefully the equipment is maintained and driven. The addition of trailers to the vehicle adds not only to the instability of the combination during heavy breaking, on corners or over bumps, but also adds to the sheer mass of the inertial body.
One of the key elements in the tandem structure is the connection between the trailers. This connection must be strong, stable, compatible with existing equipment, and easy to connect and disconnect. In addition it must take up the load of the rear trailer in such a way that it does not lift the front end of the front trailer. Also, the connection must not interfere with loading and unloading operations.
Most of the connections under consideration use the standard fifth-wheel connector, it being regarded as the best way of hitching a following trailer to a tractor or other trailer. The problem stems from the need to attach the fifth wheel to the leading trailer.
The most popular solution, is to mount the fifth wheel on a small chassis which is, in turn, mounted on a set of wheels called a bogie. This structure is pulled behind the forward trailer through a combination of poles and hooks, with the rear trailer attached to the fifth wheel. Although this idea is relatively inexpensive and is suited for use with present equipment, the connection between the structure and the leading trailer is regarded as inadequate for safety and stability. Technically, this arrangement is a true trailer rather than a semi-trailer (the proper term for a trailer not completely supported by its own road wheels), a fact which can greatly modify its operating characteristics.
All of the other solutions involve radically redesigned trailers, an idea which will not be accepted quickly due to large capital investment and the problems of acceptance by government regulatory agencies and the drivers. In addition, most of the present designs have technical drawbacks. In one plan, the chassis of the two trailers would be rigidly connected and the bogie of the forward trailer moved back to form a tandem axle at the rear of the combination (see the U.S. Pat. No. 3,102,738 of Roshia. This concept is limited to short trailer lengths, it requires that both trailers be of the new design, and it results in two non-self-supported halves should the non-conventional connection accidently come apart in transit.
The idea of attaching the rear trailer of a fifth wheel which is, in turn, connected directly to the chassis of the forward trailer, has taken several forms. In a somewhat different form of truck, an automobile carrier (see the Davis et al. U.S. Pat. No. 1,880,123) the concept of extending the chassis beyond the auto deck and attaching a fifth wheel to the extension was attempted. Since loading operations were always carried out from the rear of the rear trailer and, therefore, not impeded by the presence of the fifth wheel and since relative low-weight density of automobiles did not apparently cause the force couple of the forward trailer to result in a lifting of the front of the trailer by the weight of the rear of the trailer, the problems of transfer of the idea to a regular cargo trailer were not recognized.
In order to allow the loading and unloading of cargo, trailer designers decided that the fifth wheel must not extend beyond the end of the cargo container (see the Bennett et al. U.S. Pat. No. 3,163,306). The restricted access to a fifth wheel under the cargo container and the need for a specially designed rear trailer are obvious problems. In a refinement of the above idea, the fifth wheel and its associated bogie are slidably mounted to the frame (see the Crockett et al. U.S. Pat. No. 3,374,010). In its forward position, the fifth wheel is out of the way of the loading operation, and in the rearward position, it is accessible to a standard rear trailer. The problems of creating a practical sliding connection which is easy to operate and strong enough to withstand the tremendous stresses involved are formidable. These and other difficulties experienced with the prior art devices have been obviated in a novel manner by the present invention.
It is, therefore, an outstanding object of the invention to provide a trailer unit adapted to carry cargo behind a highway tractor in order to move goods over the road.
Another object of this invention is the provision of a trailer unit adapted to tow an additional cargo trailer using a standard fifth-wheel arrangement.
A further object of the present invention is the provision of a trailer unit in which the rear fifth-wheel unit is unencumbered by overhang of the cargo container because the trailer chassis extends beyond the cargo container.
It is another object of the instant invention to provide a trailer unit in which the rear fifth wheel is supported from directly beneath by a bogie thereby avoiding a force couple between the front and rear of the trailer.
A still further object of the invention is the provision of a trailer unit having a rigid chassis in which looseness and relative movement within itself are eliminated.
It is a further object of the invention to provide a trailer in which a gate is provided which can form a loading platform over the rear fifth wheel and behind the cargo container.
It is a still further object of the present invention to provide a trailer unit which forms a safe, stable tandem trailer which is simple to connect with another standard trailer.
With these and other objects in view, as will be apparent to those skilled in the art, the invention resides in the combination of parts set forth in the specification and covered by the claims appended hereto.